This invention relates to photosensitive devices. In particular, the present invention is an improved bond and bonding technique for bonding photodetector devices to electrically conductive substrates.
It is desirable for certain applications to bond photodetectors such as mercury cadmium telluride infrared photoconductive detectors to a heat sink in order to raise the thermal damage threshold of the detector. Heat transfer across the entire bond interface should be as high as possible.
A problem which is encountered particularly with photoconductive detectors is that the bond and the heat sink (which is usually electrically conductive) must not short out the detector. It is necessary, therefore, to provide a bond which has high thermal conductivity while providing electrical insulation between the photodetector and the metal heat sink.
One previous approach has consisted of epoxy bonding a mercury cadmium telluride photoconductive detector to a copper heat sink. The epoxy is an electrical insulator as well as a bonding material. This technique, however, is limited by the poor thermal conductivity of epoxy.
Another approach consists of evaporating an insulating layer of zinc sulfide onto the back surface of the mercury cadmium telluride photoconductive detector. A chromium layer is then deposited on the zinc sulfide, followed by a gold layer. Indium, which wets gold (but not zinc sulfide), is then used to solder the detector to the metal heat sink. The disadvantage of this approach is that multiple layers must be evaporated to achieve an insulating, solderable surface. This results in a higher probability of bonding failure due to delamination of the coatings and a higher cost due to increased fabrication steps.